This invention relates to a method for preparing dense tungsten ingots. More particularly, this invention relates to an improved method for preparing dense tungsten ingots for use in the manufacture of lamp wire.
Lamp quality tungsten wire contains small bubbles aligned in long rows parallel to the wire axis in the recrystallized tungsten filaments. Typically, although not necessarily, these bubbles contain potassium vapor. Potassium is introduced into the filament by doping tungsten powder with potassium-containing compounds and then sintering the powder to form a potassium-doped ingot. Potassium is essentially insoluble in tungsten and will reside in small pores which are refined during deformation processes to form the above-mentioned bubbles. Operation of the filament in the lamp is typically carried out at about 2900.degree. K. Potassium, which has a boiling point of about 1032.degree. K., evaporates, filling the bubbles with potassium vapor.
Cold-drawn wire undergoes recrystallization so as to convert the distorted grains in the cold-drawn wire to undistorted grains. The rows of bubbles prevent the grain boundaries in the recrystallized wire from moving perpendicular to the wire axis. The pinning of the grain boundaries in their motion provides the wire with an interlocking grain structure which results in a long-life filament. The absence of these bubbles results in grain boundary sliding and rapid failure of the filament. It is necessary, therefore, that the filament contain potassium or other material which will produce the bubbles described above.
In the current method for making dense tungsten ingots, tungsten oxide is doped with aqueous solutions of potassium disilicate and aluminum chloride. Residues of these dopants remaining on the surface of the oxide grains are removed by acid washing, for example, with hydrochloric and hydrofluoric acid. Before washing, the doped tungsten oxide is reduced to metal powder. The washed reduced tungsten powder, which contains traces of potassium, aluminum, and silicon as salts inside the individual grains, is ram pressed to form a porous green compact which is so fragile that it must be presintered at 1200.degree. C. to impart adequate structural integrity thereto. The ingot is then resistance sintered at about 3000.degree. C. to close up the porosity. During sintering, the aluminum and silicon dopants are evaporated away while much of the potassium is retained. The density of the sintered ingot is about 92% of theoretical density.
In the method described above, aluminum and silicon are necessary for retention of adequate levels of potassium during reduction of the tungsten oxide. Potassium metal is extremely volatile at reduction temperatures and cannot be incorporated into the tungsten during reduction of the oxide. Doping is achieved by adding the aluminum chloride and potassium disilicate to the tungsten oxide and then reducing the oxide, during which some of the dopants are encapsulated within the tungsten grains. The aluminum chloride and potassium disilicate react with the tungsten oxide to form high molecular weight potassium/aluminum/silicon/tungsten compounds that are stable in hydrogen at reduction temperatures and as a result are able to be incorporated within the grains. The high molecular weight potassium/aluminum/silicon/tungsten compounds decompose at the high temperatures used in sintering the tungsten powder to form the ingot. Aluminum and silicon diffuse out of the ingot and evaporate away, while much of the potassium, which is insoluble in tungsten, is retained in the form of particles residing in pores inside the ingot. Because the high molecular weight compounds formed from the dopants in the above process decompose at sintering temperatures, resulting in the loss of the aluminum and silicon, it is necessary to add the dopants prior to the formation of the metal powder in order to incorporate and retain volatile potassium in the doped tungsten.
It is to be understood that while potassium-containing dopants are used in the conventional method described above, it is known in the art that other dopants can also be used.
Aluminum and silicon, which are required in the conventional process described above, have been found to be detrimental to wire quality. As a result, it is desirable to provide a method for making dense tungsten ingots which does not use aluminum or silicon.
In the prior art method described above, most of the potassium introduced in the process in the form of potassium disilicate will be lost in the acid washing step whereby dopant residues not incorporated into the tungsten are removed from the surface thereof. Some potassium will also be lost in the sintering step. The amount of potassium which will be lost in these ways is uncertain. As a result, it is uncertain how much potassium disilicate and aluminum chloride should be doped in the tungsten oxide at the beginning of the ingot-forming process in order to obtain the desired amount of potassium in the final ingot.
It is further desirable, therefore, to provide a method for making dense tungsten ingots which provides greater certainty as to the amount of dopant which should be doped into the tungsten metal in order to obtain the desired amount of dopant in the final ingot.
A drawback to the ingot formed in the conventional method described above is the presence therein of a relatively significant gradient in potassium concentration, i.e., generally about 15 ppm of potassium with respect to tungsten, between the center and outer surface of the ingot. This gradient is a result of sintering, which provides a driving force for removal of potassium from the ingot, thereby leading to an uneven distribution of potassium in the ingot.
It is desirable to provide a method for making a dense tungsten ingot wherein such a gradient is minimized and the ingot has a relatively uniform distribution of dopant.
As mentioned above, the tungsten ingot formed in the conventional process has a density of about 92% of theoretical density. The workability of a tungsten ingot for purposes of preparing wire by rolling, swaging, and wire drawing is dependent on its density, with higher densities being preferred.
It is desirable to provide a method for making a denser tungsten ingot.
It is also desirable to provide a simpler and faster method for making a dense tungsten ingot.